1. Field of the Invention
The present invention relates to a piezoelectric ceramic suitable as a material for actuators.
2. Description of the Related Art
The piezoelectric ceramics having a basic composition expressed by a ternary Pb(Y.sub.0.5 Nb.sub.0.5)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 solid solution in which part of the Pb is substituted by Ba, Sr, etc., provide good response and are utilized as piezoelectric elements in various actuators. The ternary solid solution has proportions of the three components and an amount of the substituted Pb suitable to provide the desired piezoelectric characteristics.
Japanese Unexamined Patent Publication (Kokai) No. 57-208183 discloses a piezoelectric ceramic having a composition modified from the basic composition by adding trace amounts of a metal oxide additive, such as MgO, Sb.sub.2 O.sub.3, etc., to provide improved mechanical properties.
U.S. Pat. No. 4,882,079 discloses another piezoelectric ceramic having a composition modified from the basic composition by adding trace amounts of a metal oxide additive such as Nb.sub.2 O.sub.5, WO.sub.3, La.sub.2 O.sub.3, etc., to provide improved piezoelectric characteristics.
The above-recited conventional piezoelectric ceramics have a problem in that, when they are used as actuator elements in automobile equipment and subjected to large temperature variations, the dielectric constants of the ceramic vary as the temperature varies, causing both the current supply to a drive circuit and the rise time of the applied voltage pulse to vary with the temperature. The drive circuit must be controlled to cope with this problem.
It is therefore desired that the temperature-caused (or temperature induced) change in the dielectric constant be reduced to mitigate the load on the drive circuit.